URANIUM- AND THORIUM-DECAY NUCLIDES IN GLOBAL PHOSPHATE ROCKS AND FERTILIZERS

Since mid-1990’s, the annual global phosphate rocks production rates increased to a peak of 270 million metric tons (as P₂O₅) in 2017 and declined to a level of 220 million tons in 2021. Consistently, global phosphate fertilizer consumption has increased up to 47 million metric tons in 2020. Previous studies have reported that sedimentary phosphate rocks and associated fertilizers contain elevated levels of uranium, which could induce soil and water contamination. Here we report new data of the uranium- and thorium-decay nuclides in phosphate rocks from major global producing countries and associate fertilizers and phosphogypsum byproducts. The data show that young (<100 Ma) sedimentary phosphate ores exhibit high concentrations of U with the expected secular equilibrium between ²³⁸U and decay progeny (²²⁶Ra/²³⁸U activity ratio ~1), and predominance of U- over Th-decay series (Th/U and ²²⁸Ra/²²⁶Ra<<1). Older sedimentary phosphate rocks from China and India show lower U concentrations and higher Th/U and ²²⁸Ra/²²⁶Ra ratios. Systematic analysis of several pairs of phosphate rock and P-fertilizer as well as phosphogypsum shows, however, selective enrichment of U over Ra (²²⁶Ra/²³⁸U<1) in P-fertilizer. In contrast, phosphogypsum is characterized by Ra enrichment over U (²²⁶Ra/²³⁸U>1). This suggests that during the production of phosphate fertilizers, U and Ra partition between fertilizers and phosphogysum. The differential distribution of the uranium- and thorium decay nuclides in the products of phosphate rocks provides a novel toolkit to delineate the impact of P-fertilizers and phosphogypsum on the environment. We hypothesize that the combined low ²²⁸Ra/²²⁶Ra and ²²⁶Ra/²³⁸U activity ratios in P-fertilizers are different from the composition of common soils (²²⁸Ra/²²⁶Ra>1 and ²²⁶Ra/²³⁸U ~1). Likewise, radionuclide contamination from residual phosphogypsum would be characterized by a distinct geochemical composition (²²⁸Ra/²²⁶Ra<1 and ²²⁶Ra/²³⁸U >1). Given the elevated levels of radionuclides in phosphate rocks (e.g., ²²⁶Ra+²²⁸Ra activity in Florida phosphate rock is 1875 Bq/kg), future studies should include Ra nuclides measurements in attempts to evaluate the impact of phosphate fertilizers and phosphogypsum utilization on the quality of agriculture soils and associated water resources.